The present invention relates to a magnetic memory, more particularly, to a magnetic memory using a magnetic film with perpendicular magnetic anisotropy (PMA) as a data recording layer in each memory cell.
The magnetic memory or magnetic random access memory (MRAM) is a non-volatile memory which achieves high speed operation and infinite rewriting tolerance. This encourages practical use of MRAMs in specific applications, and promotes development for expanding the versatility of the MRAMs. A magnetic memory uses magnetic films as memory elements and stores data as the magnetization directions of the magnetic films. In writing desired data into a magnetic film, the magnetization of the magnetic film is switched into the direction corresponding to the data. Various methods have been proposed for the switching of the magnetization direction, but all of the proposed methods are the same in that a current (or write current) is used. It is of much importance to reduce the write current in realizing practical use of MRAMs. The importance of the reduction in the write current is discussed in, for example, N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, No. 4, pp. 830-838, 2007.
One approach for reducing the write current is to use “current driven domain wall motion” in data writing. As disclosed in A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, PHYSICAL REVIEW LETTERS, VOL. 92, No. 7, 077205, 2004, when a current is flown in the direction through a domain wall, the domain wall is moved in the direction of the conduction electrons. Therefore, by flowing a write current through a data recording layer, the domain wall is moved in the direction corresponding to the current direction, to thereby write desired data. An MRAM based on current driven domain wall motion is disclosed in, for example, Japanese Patent Application Publication No. 2005-191032 A.
Furthermore, a magnetic shift register based on spin injection is disclosed in U.S. Pat. No. 6,834,005. This magnetic shift register records data by using domain walls formed in a magnetic body. When a current is injected through domain walls in a magnetic body which is divided into a large number of regions (or magnetic domains), the domain walls are moved by the current. The magnetization direction of each region is defined as recorded data. Such magnetic shift register is used for, for example, recording a large amount of serial data.
It is known in the art that the write current is further reduced by using a magnetic film with perpendicular magnetic anisotropy as a data recording layer in a magnetic memory which achieves data write based on current driven domain wall motion. Such technique is disclosed in, for example, S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, JOURNAL OF APPLIED PHYSICS, VOL. 103, 07E718, (2008).
Furthermore, international publication No. WO2009/001706 A1 discloses a magnetic memory in which a magnetic film with perpendicular magnetic anisotropy is used as a data recording layer and data writing is achieved by current driven domain wall motion. FIG. 1 is a section view schematically showing a magnetoresistance effect element 200 integrated in the disclosed magnetic memory. The magnetoresistance effect element 200 includes a data recording layer 110, a spacer layer 120 and a reference layer 130. The data recording layer 110 is formed of a magnetic film with perpendicular magnetic anisotropy. The spacer layer 120 is formed of a non-magnetic dielectric layer. The reference layer 130 is formed of a magnetic layer having a fixed magnetization.
The data recording layer 110 includes a pair of magnetization fixed regions 111a and 111b, and a magnetization free region 113. The magnetization fixed regions 111a and 111b are disposed across the magnetization free region 113. The magnetizations of the magnetization fixed regions 111a and 111b are fixed in the opposite directions (or in antiparallel) by magnetization fixed layers 115a and 115b, respectively. More specifically, the magnetization direction of the magnetization fixed region 111a is fixed in the +z direction by the magnetic coupling with the magnetization fixed layer 115a, and the magnetization direction of the magnetization fixed region 111b is fixed in the −z direction by the magnetic coupling with the magnetization fixed layer 115b. The magnetization direction of the magnetization free region 113, on the other hand, is reversible between +z and −z directions by a write current which flows from one of the magnetization fixed regions 111a and 111b to the other. As a result, a domain wall 112a or 112b is formed in the data recording layer 110 depending on the magnetization direction of the magnetization free regions 113. Data are stored as the magnetization direction of the magnetization free region 113. Data may be considered as being stored as the position of the domain wall (which is indicated by the numeral 112a or 112b).
The reference layer 130, the spacer layer 120 and the magnetization free region 113 of the data recording layer 110 form a magnetic tunnel junction (MTJ). The resistance of the MTJ varies depending on the magnetization direction of the magnetization free region 113, that is, the data written into the data recording layer 110. The data are read as the magnitude of the resistance of the MTJ.
One important issue of a magnetic memory which uses a data recording layer with perpendicular magnetic anisotropy is to enhance the perpendicular magnetic anisotropy of the data recording layer. When a Co/Ni film stack (a stack in which thin Co films and Ni films are alternately laminated) is used as the data recording layer, for example strong perpendicular magnetic anisotropy can be achieved by forming the Co/Ni film stack so as to exhibit high fcc (111) orientation; however, it is not so easy to form a Co/Ni film stack with sufficiently high fcc (111) orientation.
Japanese Patent Application Publication No. 2006-114162 A discloses a perpendicular magnetic recording medium including an adhesion layer, a soft magnetic underlayer, an intermediate layer and a perpendicular recording layer, which are laminated in series over a substrate. This patent document discloses a technique for improving the magnetic characteristics and surface smoothness of the soft magnetic underlayer and for further enhancing the adhesiveness with the substrate. Specifically, the adhesion layer is composed of first and second underlayers. The first underlayer is formed of alloy of at least two elements selected from the group consisting of nickel (Ni), aluminum (Al), titanium (Ti), tantalum (Ta), chromium (Cr) and cobalt (Co), and the second underlayer is formed of metal tantalum or amorphous alloy including Ta doped with at least one element selected from the group consisting of Ni, Al, Ti, Cr and Zr.
F. J. A. den Broeder et al., “Perpendicular Magnetic Anisotropy and Coercivity of Co/Ni Multilayers”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 28, NO. 5, pp. 2760-2765, (1992) discloses that film deposition on a glass substrate without any underlayer results in strong anisotropy in the in-plane direction, and discusses that an underlayer is necessary to achieve perpendicular magnetic anisotropy. This non-patent document discloses that a gold (Au) film with (111) orientation is a preferred underlayer. It should be noted here that the underlayer disclosed in this non-patent document is formed of non-magnetic material and has a thickness of 20 nm or more.
Use of a thick non-magnetic layer as an underlayer as disclosed in this non-patent document is not preferable for the magnetic memory shown in FIG. 1, in which the magnetizations of the magnetization fixed regions of the data recording layer are fixed by the magnetization fixed layers formed under the data recording layer. When an underlayer is used in the magnetic memory shown in FIG. 1, for example, the underlayer is inserted between the data recording layer 110 and the magnetization fixed layers 115a and 115b. In this case, the magnetic coupling between the data recording layer 110 and the magnetization fixed layers 115a and 115b may be broken by the insertion of a thick non-magnetic layer as the underlayer, resulting in that the magnetizations of the magnetization fixed regions 111a and 111b are loosed. This is unpreferable for normally operating the magnetic memory.